Internal combustion engines produce exhaust gases containing a variety of pollutants, including nitrogen oxides (“NOx”), carbon monoxide, and uncombusted hydrocarbons, which are the subject of governmental legislation. Increasingly stringent national and regional legislation has lowered the amount of pollutants that can be emitted from such diesel or gasoline engines. Emission control systems are widely utilized to reduce the amount of these pollutants emitted to atmosphere, and typically achieve very high efficiencies once they reach their operating temperature (typically, 200° C. and higher). However, these systems are relatively inefficient below their operating temperature (the “cold start” period).
One exhaust gas treatment component utilized to clean exhaust gas is the NOx adsorber catalyst (or “NOx trap”). NOx adsorber catalysts are devices that adsorb NOx under lean exhaust conditions, release the adsorbed NOx under rich conditions, and reduce the released NOx to form N2. A NOx adsorber catalyst typically includes a NOx adsorbent for the storage of NOx and an oxidation/reduction catalyst.
The NOx adsorbent component is typically an alkaline earth metal, an alkali metal, a rare earth metal, or combinations thereof. These metals are typically found in the form of oxides. The oxidation/reduction catalyst is typically one or more noble metals, preferably platinum, palladium, and/or rhodium. Typically, platinum is included to perform the oxidation function and rhodium is included to perform the reduction function. The oxidation/reduction catalyst and the NOx adsorbent are typically loaded on a support material such as an inorganic oxide for use in the exhaust system.
The NOx adsorber catalyst performs three functions. First, nitric oxide reacts with oxygen to produce NO2 in the presence of the oxidation catalyst. Second, the NO2 is adsorbed by the NOx adsorbent in the form of an inorganic nitrate (for example, BaO or BaCO3 is converted to Ba(NO3)2 on the NOx adsorbent). Lastly, when the engine runs under rich conditions, the stored inorganic nitrates decompose to form NO or NO2 which are then reduced to form N2 by reaction with carbon monoxide, hydrogen and/or hydrocarbons (or via NHx or NCO intermediates) in the presence of the reduction catalyst. Typically, the nitrogen oxides are converted to nitrogen, carbon dioxide and water in the presence of heat, carbon monoxide and hydrocarbons in the exhaust stream.
Typically, NOx adsorbent materials consist of inorganic oxides such as alumina, silica, ceria, zirconia, titania, or mixed oxides which are coated with at least one platinum group metal. PCT Intl. Appl. WO 2008/047170 discloses a system wherein NOx from a lean exhaust gas is adsorbed at temperatures below 200° C. and is subsequently thermally desorbed above 200° C. The NOx adsorbent is taught to consist of palladium and a cerium oxide or a mixed oxide or composite oxide containing cerium and at least one other transition metal.
PCT Intl. Appl. WO 2004/076829 discloses an exhaust-gas purification system which includes a NOx storage catalyst arranged upstream of an SCR catalyst. The NOx storage catalyst includes at least one alkali, alkaline earth, or rare earth metal which is coated or activated with at least one platinum group metal (Pt, Pd, Rh, or Ir). A particularly preferred NOx storage catalyst is taught to include cerium oxide coated with platinum and additionally platinum as an oxidizing catalyst on a support based on aluminium oxide. EP 1027919 discloses a NOx adsorbent material that comprises a porous support material, such as alumina, zeolite, zirconia, titania, and/or lanthana, and at least 0.1 wt % precious metal (Pt, Pd, and/or Rh). Platinum carried on alumina is exemplified.
NOx adsorber catalysts are typically exposed to cyclic oxidizing and reducing conditions. During the oxidizing cycle, NOx is stored on the catalyst. During the reducing cycle, NOx is released and/or reduced (e.g. to N2). It is well known in the art (Applied Catalysis B: Environmental 5 (1995) 367-376) that certain catalysts, such as platinum on ceria (Pt—CeO2), can be activated by a reduction pre-treatment. However, such activation can only be achieved above a relatively high temperature, and is reversed after re-oxidation pre-treatment.
At low temperatures (typically below about 200° C.), the NOx storage function of these catalysts is inefficient and continues to be an area of catalyst development in need of improvement. It is also desirable for catalysts to be developed that have little or no NOx storage properties at greater than a specific temperature, to allow control of when NOx is released for subsequent conversion by, for example, a further downstream catalyst.
As with any automotive system and process, it is desirable to attain still further improvements in exhaust gas treatment systems. We have discovered a new NOx adsorber catalyst composition with improved low temperature NOx storage characteristics, and improved NOx release properties.
It has surprisingly been found that NOx adsorber catalyst compositions comprising platinum group metals on a support that is doped with a mixture of certain metals or metal oxides have improved low temperature NOx storage properties, and which, in certain embodiments, are more readily activated by reduction pre-treatment and retain this activation after exposure to oxidizing conditions.